Dimerization is necessary for MIM-mediated membrane deformation and endocytosis

CRY-BARs: Versatile light-gated molecular tools for the remodeling of membrane architectures

BAR (Bin, Amphiphysin and Rvs) protein domains are responsible for the generation of membrane curvature and represent a critical mechanical component of cellular functions. Thus, BAR domains have great potential as components of membrane-remodeling tools for cell biologists. In this work, we describe the design and implementation of a family of versatile light-gated I-BAR (inverse-BAR) domain containing tools derived from the fusion of the A. thaliana Cryptochrome 2 photoreceptor and I-BAR protein domains ('CRY-BARs') with applications in the remodeling of membrane architectures and the control of cellular dynamics. By taking advantage of the intrinsic membrane binding propensity of the I-BAR domain, CRY-BARs can be used for spatial and temporal control of cellular processes that require induction of membrane protrusions. Using cell lines and primary neuron cultures, we demonstrate here that the CRY-BAR optogenetic tool evokes membrane dynamics changes associated with cellular activity. Moreover, we provide evidence that ezrin, an actin and PIP₂ binding protein, acts as a relay between the plasma membrane and the actin cytoskeleton and therefore is an important mediator of switch function. Overall, we propose that CRY-BARs hold promise as a useful addition to the optogenetic toolkit to study membrane remodeling in live cells.

Missing-in-Metastasis/Metastasis Suppressor 1 Regulates B Cell Receptor Signaling, B Cell Metabolic Potential, and T Cell-Independent Immune Responses

Efficient generation of antibodies by B cells is one of the prerequisites of protective immunity. B cell activation by cognate antigens via B cell receptors (BCRs), or pathogen-associated molecules through pattern-recognition receptors, such as Toll-like receptors (TLRs), leads to transcriptional and metabolic changes that ultimately transform B cells into antibody-producing plasma cells or memory cells. BCR signaling and a number of steps downstream of it rely on coordinated action of cellular membranes and the actin cytoskeleton, tightly controlled by concerted action of multiple regulatory proteins, some of them exclusive to B cells. Here, we dissect the role of Missing-In-Metastasis (MIM), or Metastasis suppressor 1 (MTSS1), a cancer-associated membrane and actin cytoskeleton regulating protein, in B cell-mediated immunity by taking advantage of MIM knockout mouse strain. We show undisturbed B cell development and largely normal composition of B cell compartments in the periphery. Interestingly, we found that MIM-/- B cells are defected in BCR signaling in response to surface-bound antigens but, on the other hand, show increased metabolic activity after stimulation with LPS or CpG. In vivo, MIM knockout animals exhibit impaired IgM antibody responses to immunization with T cell-independent antigen. This study provides the first comprehensive characterization of MIM in B cells, demonstrates its regulatory role for B cell-mediated immunity, as well as proposes new functions for MIM in tuning receptor signaling and cellular metabolism, processes, which may also contribute to the poorly understood functions of MIM in cancer.

Missing-in-metastasis protein promotes internalization of magnetic nanoparticles via association with clathrin light chain and Rab7

BACKGROUND

Magnetic nanoparticles (MNPs) have been widely used in biomedical applications. Proper control of the duration of MNPs in circulation promises to improve further their applications, in particularly drug delivery. It is known that the uptake of tissue-associated MNPs is mainly carried out by macrophages. Yet, the molecular mechanism to control MNPs internalization in macrophages remains to be elusive. Missing-in-metastasis (MIM) is a scaffolding protein that is highly expressed in macrophages and regulates receptor-mediated endocytosis. We hypothesize that uptake of MNPs may also involve the function of MIM.

METHODS

We investigated the effect of MIM expression on the intracellular trafficking of MNPs by transmission electronic microscopy, flow cytometry, o-phenanthroline photometric analysis, Perl's staining, immunofluorescence microscopy and co-immunoprecipitation. To explore the molecular events in MIM-mediated MNPs uptake, we examined the effect of MNPs on the interaction of MIM with clathrin, Rab5 and Rab7.

RESULTS

Uptake of MNPs was significantly enhanced in cells overexpressing MIM. Upon exposure to MNPs, MIM was associated with clathrin light chain in endocytic vesicles and Rab7, a protein that regulates late endosomes. However, MNPs caused dissociation of MIM with Rab5, an early endosome-associated protein.

CONCLUSIONS

MIM regulates internalization of MNPs via promoting their trafficking from plasma membrane to late endosomes.

GENERAL SIGNIFICANCE

Our data unveiled a novel pathway which MNPs internalization and intracellular trafficking in macrophages. This new pathway may allow us to control the uptake of MNPs within cells by targeting MIM, thereby improving their medical applications.

Role of the Scaffold Protein MIM in the Actin-Dependent Regulation of Epithelial Sodium Channels (ENaC)

Epithelial Sodium Channels (ENaCs) are expressed in different organs and tissues, particularly in the cortical collecting duct (CCD) in the kidney, where they fine tune sodium reabsorption. Dynamic rearrangements of the cytoskeleton are one of the common mechanisms of ENaC activity regulation. In our previous studies, we showed that the actin-binding proteins cortactin and Arp2/3 complex are involved in the cytoskeleton-dependent regulation of ENaC and that their cooperative work decreases a channel's probability of remaining open; however, the specific mechanism of interaction between actin-binding proteins and ENaC is unclear. In this study, we propose a new component for the protein machinery involved in the regulation of ENaC, the missing-in-metastasis (MIM) protein. The MIM protein contains an IMD domain (for interaction with PIP₂ -rich plasma membrane regions and Rac GTPases; this domain also possesses F-actin bundling activity), a PRD domain (for interaction with cortactin), and a WH2 domain (interaction with G-actin). The patch-clamp electrophysiological technique in whole-cell configuration was used to test the involvement of MIM in the actin-dependent regulation of ENaC. Co-transfection of ENaC subunits with the wild-type MIM protein (or its mutant forms) caused a significant reduction in ENaC-mediated integral ion currents. The analysis of the F-actin structure after the transfection of MIM plasmids showed the important role played by the domains PRD and WH2 of the MIM protein in cytoskeletal rearrangements. These results suggest that the MIM protein may be a part of the complex of actin-binding proteins which is responsible for the actin-dependent regulation of ENaC in the CCD.

Missing-in-metastasis protein downregulates CXCR4 by promoting ubiquitylation and interaction with small Rab GTPases

Surface expression of chemokine receptor CXCR4 is downregulated by missing-in-metastasis protein (MIM; also known as MTSS1), a member of the inverse BAR (I-BAR)-domain protein family that recognizes and generates membranes with negative curvature. Yet, the mechanism for the regulation is unknown. Here, we show that MIM forms a complex with CXCR4 by binding to E3 ubiquitin ligase AIP4 (also known as ITCH) in response to stromal cell-derived factor 1 (SDF-1; also known as CXCL12). Overexpression of MIM promoted CXCR4 ubiquitylation, inhibited cellular response to SDF-1, caused accumulation and aggregation of multivesicular bodies (MVBs) in the cytoplasm, and promoted CXCR4 sorting into MVBs in a manner depending on binding to AIP4. In response to SDF-1, MIM also bound transiently to the small GTPase Rab5 at 5 min and to Rab7 at 30 min. Binding to Rab7 requires an N-terminal coiled-coil motif, deletion of which abolished MIM-mediated MVB formation and CXCR4 internalization. Our results unveil a previously unknown property of MIM that establishes the linkage of protein ubiquitylation with Rab-guided trafficking of CXCR4 in endocytic vesicles.

The SH3 domain distinguishes the role of I-BAR proteins IRTKS and MIM in chemotactic response to serum

The family of inverse BAR (I-BAR) domain proteins participates in a range of cellular processes associated with membrane dynamics and consists of five distinct members. Three of the I-BAR proteins, including insulin receptor tyrosine kinase substrate (IRTKS), contain an SH3 domain near their C-termini. Yet, the function of the SH3 domain of IRTKS remains uncharacterized. Here we report that in contrast to MIM, which is a prototype of I-BAR proteins and does not contain an SH3 domain, IRTKS promoted serum-induced cell migration along with enhanced phosphorylation of mitogen activated kinases Erk1/2 and p38, and activation of small GTPases Rac1 and Cdc42. In addition, cells overexpressing IRTKS exhibited an increased polarity characterized by elongated cytoplasm and extensive lamellipodia at leading edges. However, a mutant with deletion of the SH3 domain attenuated both cellular motility and p38 phosphorylation but had little effect on Erk1/2 phosphorylation. Also, a chimeric mutant in which the N-terminal portion of MIM is fused with the C-terminal IRTKS, including the SH3 domain, was able to promote chemotactic response to serum and cellular polarity. In contrast, a chimeric mutant in which the N-terminal IRTKS is fused with the C-terminal MIM failed to do so. Furthermore, treatment of cells with SB203580, a selective inhibitor of p38, also neutralized the effect of IRTKS on cell migration. These data indicate that the SH3 domain distinguishes the function of IRTKS in promoting cell migration and inducing signal transduction from those of SH3-less I-BAR proteins.

Downregulation of MIM protein inhibits the cellular endocytosis process of magnetic nanoparticles in macrophages

MIM plays a positive role in the RAW 264.7 cellular endocytosis process of iron oxide nanoparticles mainly in clathrin-mediated pathway, which is a meaningful molecular basis for biomedical applications of nanomaterials.